Bearings or half shafts or what?

Trying to eliminate a moaning noise that increases with speed (not RPM) on a 190E 2.6L 5-Sp with 249,000 miles on it. Noise seems to come from the rear, regardless of in gear or in neutral - thus it is a function of MPH. A used differential was installed recently, so I don't think that is the problem (the noise was there before the used diff. was installed). New flex discs and driveshaft center bearing installed, too. If it is the rear bearings, would they make a moan noise or would it be a higher pitched noise (like a chirp that you get when the front bearings are bad)? Could it be half shafts? But I thought when half shafts go bad, they make a Click/Clack noise. Any and all advice is welcome. Thank you.